How to hide/redirect the author page

You can do this at an earlier moment by hooking into the right action, like template_redirect, which fires right before the template will be displayed.

add_action( 'template_redirect', 'wpse14047_template_redirect' );
function wpse14047_template_redirect()
{
    if ( is_author() ) {
        $id = get_query_var( 'author' );
        // get_usernumposts() is deprecated since 3.0
        $post_count = count_user_posts( $id );
        if ( $post_count <= 0 ) { 
            //This line could also be wp_redirect 
            include( STYLESHEETPATH .'/author-redirect.php' );
            exit;
        }
    }
}